1. Field of the Invention
The present invention relates to a fixer and an image forming apparatus including the fixer, and more particularly to a fixer and an electronographic image forming apparatus including the fixer.
2. Discussion of the Background
In general, an electronographic image forming apparatus such as a copying machine, a printer, and a facsimile machine may include an image forming mechanism for forming an image, e.g., a toner image, on a recording medium such as a sheet or an OHP film, and a fixer to fix the toner image on the recording medium.
An example of a fixer includes a fixing member, a heat source to heat the fixing member, and a pressurizer. The pressurizer is pressed to the fixing member to form a fixing nip therebetween. When the recording medium passes through the fixing nip, the toner image is fused and fixed with heat from the fixing member and pressure from the pressurizer onto the recording medium. The fixing member may be a roller in which a heat source is provided. Alternatively, the fixing member may be a belt wound around a roller having a heat source therein. As a heat source, heat from a halogen heater provided near the heating member may be used.
The heat source of the above fixer may be turned off during waiting time to save energy. When an image formation is started, the heat source is turned on and the fixing member is heated to a fixing temperature during warm-up time (startup time). To keep power consumption low to save energy, the fixing member desirably has a lower heat capacity.
A fixer employs an induction heating method to shorten the warm-up time and to save energy. For example, an induction heating fixer 101 includes a fixing roller 3, a pressing roller 4, and an induction coil 5 as illustrated in FIG. 1. The fixing roller 3 includes a support shaft 1 in its center and a fixing rotator 2 wound around the support shaft 1. The pressing roller 4 includes a core metal 6 and a rubber layer 7 around the core metal 6. The pressing roller 4 is pressed against the fixing roller 3 and a nip n is formed between the two rollers. The induction coil 5 is provided around the fixing roller 3 in a non-contact manner.
FIG. 2 illustrates edges of the fixing roller 3 and the pressing roller 4 in their width direction (axis direction). The support shaft 1 includes a core metal 1a and an elastic insulating layer 1b wound on the core metal 1a. With the elastic insulating layer 1b, the nip n may have a sufficient width for fixing. The fixing rotator 2 includes an induction heating layer 2a, an elastic layer 2b, and a release layer 2c from inside.
When electric current is applied to the induction coil 5 (shown in FIG. 1), a magnetic field of high-frequency waves is induced. Induction current occurs at a side near the induction coil 5 in the induction heating layer 2a. An outer surface of the fixing roller 3 is heated with joule heating.
An optimum input power was determined by adjusting an eddy-current load on the fixing rotator 2. The graph of FIG. 3 shows a relation between heating value and eddy-current load at a frequency of 30 kHz. The eddy-current load specifies heating characteristics of a heating layer by induction heating and may be expressed in the following formula:d=vr/t 
wherein d is the eddy-current load, vr is a volume resistivity of the heating layer, and t is a thickness of the heating layer.
However, magnetic flux may penetrate only to a depth less than an epidermis depth δ, when the thickness of the heating layer is larger than the epidermis depth δ. In that case, the eddy-current load may be expressed in the following formula:d=vr/δ
When k is a constant, ρ is a resistivity, μ is a relative permeability, and f is a frequency, the epidermis depth δ may be expressed in the following formula:δ=k (ρ/fμ)1/2 
Based on the above, a thickness of the induction heating layer 2a at which the eddy-current load was optimum was checked by inductively heating the induction heating layer 2a that includes a material whose resistivity is lower at a frequency of around 30 kHz. The thickness of the induction heating layer 2a was less than a few dozen micron meters which was remarkably thinner than a thickness of a thin sleeve of a halogen heater that was a few hundred micron meters.
However, when the induction heating layer 2a is thinner, the fixing rotator 2 consequently becomes thinner. The geometrical moment of inertia is in proportion to a third power of the thickness. Therefore, mechanical strength (flexural rigidity) is decreased when the fixing rotator 2 becomes thinner. The flexural rigidity is expressed in the following formula:Flexural rigidity=E×I wherein E is modulus of direct elasticity and I is the geometrical moment of inertia.
When the pressing roller 4 is pressed to the fixing roller 3 as illustrated in FIG. 1, the elastic insulating layer 1b in the support shaft 1 is elastically deformed. Due to elastic repulsion of the elastic insulating layer 1b, both edges e of the fixing roller 3 in its width direction shown as arrow T are likely to curve as illustrated by a two-dot chain line in FIG. 2. Further, a shearing force occurs in the sleeve-shaped fixing rotator 2, which may damage the fixing rotator 2.
To prevent such damage, it is necessary to increase the thickness of the fixing rotator 2. However, when the fixing rotator 2 is thicker, flexibility of the fixing rotator is decreased. Consequently, it may be difficult to form enough of a fixing nip n.